1. Field of the Invention
The present invention relates to a process for producing a motor vehicle body part. More particularly, the present invention is directed to a motor vehicle roof built in a sandwich construction having layers from an outside shell which consists of a foam layer with an outside skin, and an inside shell. The inside shell is inserted as a prefabricated component into a first mold half of a mold tool, with foam material being applied to the shaped outer skin which is placed in the second mold half of the mold tool, the two mold halves being closed and the foam material being allowed to cure at an elevated temperature.
2. Description of the Related Art
Motor vehicle body parts in a sandwich construction of the type in accordance to the present invention allow for custom vehicle individualization. Applied to a motor vehicle roof in this way the replacement of a conventional motor vehicle roof skin by a modular roof system is made possible. For example, one process allows the motor vehicle manufacturer to produce a modular roof system more quickly and economically in an effort to react to market developments. One such modular roof is disclosed in DE 197 09 016 A1, which discloses a modular roof in a one-piece inseparable execution comprising an inside shell and an outside shell which are produced separately and thereupon integrated. The outside skin of the outside shell of the motor vehicle body part is prepared in a sandwich construction and is conventionally deep drawn from a foil or sheet metal and is foamed with a foam, for example polyurethane, by which the outside shell is completed. Finally, the separately produced inside shell is inserted into one half of the mold of a foaming tool, and the outside shell is positioned into the second half. Thereupon the foaming tool is closed and the body part in a sandwich construction is cured and thereupon removed from the foaming tool as an installable prefabricated component. In this process, however, two separate tools are needed for production, specifically a deep-drawing tool and a foaming tool; thereby resulting in high tool costs and increased production time.
German Patent DE 36 04 175 C2 discloses a process and a means for the mold foaming of foam in conjunction with a hard plastic. This process requires that the hard plastic be placed into contact with one mold half of a mold tool to be pre-molded before mold foaming of the foam by the hard plastic fixed on the mold half. The mold open is pre-molded using a movable plunger which is provided on this mold half, whereupon the foam is foam-molded by application of steam pulse to the hard plastic. This process uses a single tool for producing a foam molding with an outside skin, however, has the disadvantage in that one of the two mold halves is constructed relatively complex with a plunger and a plunger drive and must be actuated in a correspondingly complex manner.
Accordingly, it is an object of the present invention to reduce the cost in the execution of the initially mentioned process by providing a simple and reliable process for producing a motor vehicle body part in a sandwich construction.
The process in accordance to the present invention is based upon the use of a single mold tool and is carried out such that with two successive tool closing strokes where initially the outside skin of the outside shell is molded and thereupon with a second closing stroke the foam layer of the outside shell is formed and the integration of the outside shell and inside shell is effectuated. In this two-stage production of the motor vehicle body part in the same molding tool, shaping of the outside skin takes place in the first stage with a maximum stroke of the two mold halves, while the foam body is produced and the outside shell and the inside shell are integrated with a reduced stroke in the second stage. This reduced stroke leaves a cavity for the foaming process for producing the desired thickness of the foam layer of the outside shell. By means of the conventional thermal effect, the foam material for the foam layer expands in this cavity and forms a permanent binder between the outside shell and the inside shell as soon as the foam layer has completely cured.
In the preferred embodiment, polyurethane is used as the foam material. The foam material can optionally contain portions of glass fiber to increase the strength of the foam layer. For the outside skin of the outside shell, any flat materials can be used provided they can be formed and exhibit the required strength. For instance, one suitable material for the outside skin is accordingly plastic both in the form of a film and sheet.
In the following, embodiments of the invention are explained by way of example with reference to the drawings.